A New Energy-Efficient and High Throughput Two-Phase Multi-Bit per Cycle Ring Oscillator-Based True Random Number Generator

Abstract

Oscillator-based elementary true random number generator (TRNG) uses a slow jittery ring oscillator (RO) to sample a fast RO. The ROs are always on but most of the oscillatory cycles of the fast RO are not sampled into random bits. In this paper, a new lightweight TRNG design is proposed to minimize the power wasted by the superfluous oscillations. Random bits are extracted from both phases of the slow ROs to increase the throughput and the fast RO is activated only during the narrow transition time difference between two symmetrically designed slow ROs. The slow jittery ROs are implemented using current starved inverters biased in the weak inversion region to reduce their power consumption. Their jitter amplitudes are increased by lowering the oscillation frequency and reducing the drain current of the transistors. The narrow jittery pulse generated by the differential pair of slow ROs is quantized by the fastest three-stage RO. Two random bits from each phase of the jittery ROs can be extracted by using a gigahertz dynamic toggled D flip-flop counter to count the number of oscillatory cycles of the fast RO. The proposed TRNG is fabricated in a standard 65 nm 1.2 V CMOS process. Measurement results of the fabricated chips show that the proposed TRNG consumes merely 260 μW at a bit rate of 52 Mbps. It outperforms the state-of-art on-chip jitter-based TRNGs with the best figure-of-merit of 5 pJ/bit and the smallest footprint of 366 μm². Its generated bit sequence passes the statistical randomness tests including National Institute of Standards and Technology (NIST) test, Auto Correlation Factor (ACF) test and bias. The mean redundancy of the ten tested chips is measured to be less than 10⁻⁵ bit/symbol.